This invention relates to cavity filters for filtering electromagnetic signals and, more particularly, to a filter constructed of a cavity having internal ridges which enable multiple mode operation of the filter and permit a reduction in the physical size of the cavity.
Microwave filters are employed in numerous signal processing situations ranging from satellite communication systems to radar systems. The use of microwave cavity filters in conjunction with a phased array antenna, carried by a satellite in a communication system, is of particular interest herein. Such filters may be employed to filter incoming and outgoing signals, and may be used in the construction of a diplexer.
The physical sizes of the cavities of such filters vary in accordance with the wavelength of the microwave signals to be filtered, with longer wavelength signals requiring larger cavities and shorter wavelength signals requiring smaller cavities. In the case of cavity filters carried by satellites, it is particularly important to reduce the overall size of the filter to facilitate the integration of the filter with other components of the satellite. A reduction on size can be accomplished by use of cavities operable with multiple modes of electromagnetic signals within the cavities. For example, a cavity of a filter operable in two orthogonal modes can produce the filter passband characteristics of a two-cavity filter with a single cavity.
However, a problem exists in that the foregoing reduction in filter size does not suffice for satellite systems operating at lower microwave frequencies such as L-band and S-band. The physical sizes of multiple pole filters having short cut-off passband characteristics, particularly in the situation wherein two such filters are employed in a diplexer connected to antenna elements, present substantial difficulty in packaging all of the microwave components within the region of space allocated for a phased array antenna.